Check for count == 0 when computing center of mass.

Bug: oss-fuzz:41243
Change-Id: I0297469057826f83ab35c43733ffb7d3cc5b9965
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/508679
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>

src/gpu/ops/AAConvexPathRenderer.cpp

@@ -71,6 +71,9 @@ bool center_of_mass(const SegmentArray& segments, SkPoint* c) {
     SkScalar area = 0;
     SkPoint center = {0, 0};
     int count = segments.count();
+    if (count <= 0) {
+        return false;
+    }
     SkPoint p0 = {0, 0};
     if (count > 2) {
         // We translate the polygon so that the first point is at the origin.